A number of methods have heretofore been used to make steel parts and structural members. These methods often begin with bars of high-strength material and employ cold forming techniques, such as rolling, upsetting, heading and extrusion, which are well known in the art. In upsetting, the cross-sectional area of a portion or all of a bar of metal is increased. Heading is a particular form of upsetting where the starting material is wire, rod or bar stock. The heads of bolts are often made using heading techniques. In extrusion, the metal bar is forced through a die orifice of a desired cross-sectional outline to produce a length of metal having a uniform cross section. Extrusion is particularly applicable for forming elongate structural members having a uniform cross-sectional configuration over substantially the entire length of the member. Rolling includes forming a finished member by repeatedly passing rollers over the length of the bar until it is formed into the desired shape.
One such method for making high-strength steel structural members which is well known begins by annealing or otherwise softening the steel bar. The annealed steel bar is then cold formed, in a process which includes one of the above described forming techniques, into a desired geometric cross-section. The now formed structural member is then heat treated, i.e., austenitized, hardened by quenching followed by tempering, to obtain the high-strength mechanical properties desired. The steel material of the resulting member typically has a tempered martensite microstructure. The mechanical properties produced from such heat treatments are often inconsistent and can vary widely from member to member. In addition, the annealing and heat treating steps significantly add to the cost of the overall process for making the high-strength steel structural members, due in large part to the energy consumption associated with heating the member and the required labor and processing.
In another method for making such high-strength steel structural members, the steel is initially austenitized, hardened by quenching and then tempered to the point where the mechanical properties of the post-heat treated bar are such that it can be subsequently cold formed, in a process which includes one of the above described forming techniques, into a desired geometric cross-section. The steel material of the finished member from this method also has a tempered martensite microstructure. While this method apparently has advantages over the previously described method in that narrower strength tolerances from member to member have reportedly been obtained, this method still employs a costly heat treating process.
Cold forming high-strength material is known. In U.S. Pat. No. 3,904,445, hereby incorporated by reference in its entirety, which issued to the present assignee, a method is disclosed for cold forming a length of high-strength steel bar stock into a U-bolt. However, cold forming a bend in a length of bar stock is less severe than other cold forming techniques, such as upsetting and extruding. Until the invention of the '445 patent, it was thought that cold forming a blank of high-strength into a part or structural member by upsetting or extrusion type techniques would likely result in the formation of cracks or even fractures in the finished product or at the least would likely require the gradual formation of the member by a series of cold forming steps with an annealing or stress relieving step performed between successive cold forming operations. Such cracks or fractures would likely ruin the member. In addition, employing such cold forming and annealing steps would add to the time and cost of making such high strength steel structural members.
One newer method for cold forming high-strength steel structural members is disclosed in U.S. Pat. No. 5,496,425, hereby incorporated by reference in its entirety, which issued to the present assignee. In the practice of the invention described in the '425 patent, high-strength steel material having a specific chemical composition is cold formed into a structural member for forging or extruding the high-strength steel material through a tapered die is required as in typical forging and extrusion processes. While such a process avoids many of the disadvantages and drawbacks described hereinabove and associated with warm or hot forming of structural members, it does require the application of significant forces and pressures associated with the extrusion process. Specifically, forcing high-strength steel material in a cold drawing process through a tapered die or the like to form a structural member requires a significant amount of pressure or energy to be exerted on the steel material, the die and associated machinery. As such, forging and extrusion processes for cold forming structural members require a significant amount of energy and may result in damage to the forging or extrusion equipment as well as frequent replacement of the dies or associated components.
A die suitable for cold drawing or forging process is very costly and therefore a significant and potentially expensive item for repair and replacement. Therefore, the opportunity to avoid cold drawing or extrusion offers significant advantages in the commercial production of high-strength steel structural members. Additionally, the capacity for heat-treating structural members to increase or improve the mechanical properties is limited. Therefore, the requirement for such heat treatment should, if at all possible, be avoided while still providing high-strength steel structural members with the appropriate strength levels.